The resent invention relates to an improved pump for a pumping plant for low pressure, self-contained oil-filled cables. It has been common practice to use so-called "canned", electrically driven pumps to pump oil from a storage tank and into the cable. It is particularly important to have a gas tight system in the pumping plant since the oil in the storage tank has to be kept under vacuum at all times to keep the oil in a gas-free condition.
Reciprocating pumps have, however, shown a number of advantages above an electrically driven screw -- or gear pump, especially those where the pump is air driven, since maintaining a certain pressure on the cable system only means supplying the pump with air (or gas) at a certain pressure, and because the pump will only operate when oil is needed to maintain the desired pressure on the cable system.
At least one such air driven pump, namely one marketed under the trade name Haskel, has proven itself very useful for this purpose. Even though a pumping plant using such a pump is no longer "hermetically sealed", since piston rod seals have to be used, these seals have proven very effective in preventing air and moisture from entering the system.
To improve the conditions even further, it is known (Norw. Patent No. 160.318, S. Ege 13-2-1) to maintain a area between the two piston rod seals filled with oil from an oil cup, and it is even common to place another oil cup in the area outside the outer seal. The function of the latter cup is primarily that of providing better lubrication at the outer seal. The oil in these two cups will, however, after some time no longer be degasified, and there is a remote possibility that a minute amount of oil from the area in between the two seals may be drawn past the inner seal by the piston rod, and after a considerable amount of time cause a slight contamination of the oil in the system.